1. Field of the Invention
The present invention relates to an angular velocity sensor for detecting the angular velocity of a rotating body.
2. Description of the Related Arts
In operation of an angular velocity sensor,the vibration plate of the angular velocity sensor is vibrated in any direction selected from three directions including X, Y, and Z directions. When the vibration plate is vibrating with a constant amplitude for example in the X direction, if the angular velocity sensor is rotated about the Z axis, a Coriolis force (inertia force) acts on the vibration plate, which causes the vibration plate to be displaced in the Y direction. The above displacement of the vibration plate in the Y direction caused by the Coriolis force is detected by detecting the change in capacitance or piezoresistance. Various angular velocity sensors, one-side or both-side supporting type, such as those disclosed in Japanese Unexamined Patent Publications Nos. 61-139719, 61-114123 and 6-123632 are widely used.
Of these various conventional angular velocity sensors, the angular velocity sensor disclosed in Japanese Unexamined Patent Publication No. 6-123632 is shown in FIGS. 17 and 18.
As shown in FIGS. 17 and 18, the angular velocity sensor 1 has a substrate 2 or a main body formed into a rectangular shape. The substrate 2 is made up for example of silicon having a high resistance.
A movable member 3 is disposed on the substrate 2, wherein the movable member 3 is formed of low-resistance polysilicon or single-crystal silicon doped with P, B, or Sb. The movable member 3 includes: four fixing parts 4, 4, . . . formed on the substrate 2, at the respective four corners; four L-shaped supporting beams 5, 5, . . . each extending from the corresponding supporting element 4 and each having a part extending in parallel to the X axis and a part extending in parallel to the Y axis; and a vibration plate 6 which is supported by the four supporting beams 5 such that the vibration plate 6 is spaced from the surface of the substrate 2 and can move in the X and Y directions. Movable comb-shaped vibration-urging electrodes 7, 7 are formed on the front and rear side faces, respectively, of the vibration plate 6, wherein the front and rear side faces refer to those side faces perpendicular to the X direction. Each movable comb-shaped vibration-urging electrode 7 includes a plurality of electrode plates 7A, 7A, . . . (four electrode plates in the specific example shown in FIGS. 17 and 18) projecting from each of these side faces. Movable comb-shaped detection electrodes 8, 8 are formed on the left and right side faces, respectively, of the vibration plate 6, wherein the left and right side faces refer to those side faces perpendicular to the Y direction. Each movable comb-shaped detection electrode 8 includes a plurality of electrode plates 8A, 8A, . . . (four electrode plates in the specific example shown in FIGS. 17 and 18) projecting from each of these side faces.
Of various portions of the movable member 3, only the fixing parts 4 are firmly connected to the substrate 2, and the other portions including the vibration plate 6 and the supporting beams 5 are supported at four points such that they are spaced a predetermined distance apart from the substrate 2. Since the vibration plate 6 is supported by the supporting beam 5 each having an L shape, the vibration plate 6 can move in the X direction with the part parallel to the Y axis of the L-shaped supporting beam 5 being bent. Similarly, the vibration plate 6 can move in the Y direction with the part parallel to the X axis of the L-shaped supporting beam 5 being bent.
The angular velocity sensor 1 also has a pair of fixed comb-shaped vibration-urging electrodes 9,9 which are formed on the substrate 2 at the front and rear sides of the vibration plate 6 such that the vibration plate 6 is interposed between these fixed comb-shaped vibration-urging electrodes. Each fixed comb-shaped vibration-urging electrode 9 includes: a fixed part 9A formed on the substrate 2, in front of or at the back of the vibration plate 6; and four electrode plates 9B, 9B, . . . extending from each fixed part 9A in parallel to and apart from the corresponding electrode plates 7A of the movable comb-shaped vibration-urging electrode 7.
The angular velocity sensor 1 also has a pair of fixed comb-shaped detection electrodes 10,10 which are formed on the substrate 2 at its left and right sides, respectively, such that the vibration plate 6 is interposed between these fixed comb-shaped detection electrodes. Each fixed comb-shaped detection electrode 10 includes: a fixed part 10A formed on the substrate 2, adjacent to the left or right end of the vibration plate 6; and four electrode plates 10B, 10B, . . . extending from each fixed part 10A in parallel to and apart from the corresponding electrode plates 8A of the movable comb-shaped detection electrode 8.
Reference numerals 11,11 denote two vibration generating parts serving as vibration generating means each formed with one movable comb-shaped vibration-urging electrode 7 and one fixed comb-shaped vibration-urging electrode 9. The electrode plates 7A of each movable comb-shaped vibration-urging electrode 7 are equally spaced from the respective electrode plates 9B of the corresponding fixed comb-shaped vibration-urging electrode 9. If vibration driving signals with a frequency f are applied between the movable comb-shaped vibration-urging electrodes 7 and the fixed comb-shaped vibration-urging electrodes 9 such that the signals applied to electrodes at the front and left sides are opposite in phase to each other, then electrostatic attraction occurs between the electrode plates 7A and 9B alternately at the front and rear sides, and thus attraction and repulsion occur alternately and periodically at each vibration generating part 11. As a result, the vibration plate 6 vibrates in a direction denoted by the arrow a, i.e., the X direction.
Reference numerals 12,12 denote two displacement detecting parts serving as displacement detecting means each formed with one movable comb-shaped detection electrode 8 and one fixed comb-shaped detection electrode 10. There are provided gaps do between the electrode plates 8A of the movable comb-shaped detection electrode 8 and the adjacent electrode plates 10B of the fixed comb-shaped detection electrode 10. The above electrodes 8 and 10 form a parallel-plate detection capacitor. Each displacement detecting part 12 detects the change in the effective area between the electrode plates 8A and 10B by detecting the corresponding change in capacitance.
In the angular velocity sensor 1 having the above structure, if vibration driving signals having opposite phases are applied to the respective vibration generating parts 11, then electrostatic attraction between the electrode plates 7A and 9B occurs alternately in the vibration generating parts 11 at the front and rear sides. As a result the vibration plate 6 is displaced alternately forward and backward in the X direction denoted by the arrow a, and thus the vibration plate 6 vibrates in the X direction.
When the angular velocity sensor 1 is vibrating in the above-described manner, if an angular velocity n about the Z axis is applied to the angular velocity sensor 1, a Coriolis force (inertia force) having a magnitude given by equation 2 shown below is generated in the Y direction. The Coriolis force causes the vibration plate 6 to vibrate in the Y direction.
In the vibrating operation of the vibration plate 6 generated by the vibration generating parts 11, the displacement x in the X direction and its velocity V are given by: EQU x=A.multidot.sin ((2.pi.f)t) EQU V=A (2.pi.f) cos ((2.pi.f)t) (1)
where A is the amplitude of the vibration of the vibration plate 6, and f is the frequency of the vibration driving signal.
The Coriolis force F, which occurs when an angular velocity .OMEGA. about the Z axis is applied to the vibration plate 6 vibrating with the displacement x and the velocity V described above, is given as: ##EQU1## where m is the mass of the vibration plate 6.
The Coriolis force F represented by equation 2 causes the vibration plate 6 to vibrate in the Y direction. The vibrating displacement of the vibration plate 6 results in a change in the capacitance between the movable comb-shaped detection electrode 8 and the fixed comb-shaped detection electrode 10 in each displacement detecting part 12. Thus the angular velocity .OMEGA. about the Z axis can be determined by detecting the above change in the capacitance.
As described above, each vibration generating part 11 is formed with the electrode plates 7A of the movable comb-shaped vibration-urging electrode 7 and the electrode plates 9B of the fixed comb-shaped vibration-urging electrode 9 such that the electrode plates 7 and 9 can effectively have a large facing area, which allows the electrode plates 7A and 9B to have a great electrostatic attractive force in response to the vibration driving signal applied to each vibration generating part 11 thereby allowing the vibration plate 6 to vibrate with a large amplitude in the direction denoted by the arrow a.
Similarly, each displacement detecting part 12 is formed with the electrode plates 8A of the movable comb-shaped detection electrode 8 and the electrode plates 10B of the fixed comb-shaped detection electrode 10 such that the electrodes 8 and 10 effectively have a large facing area. As a result, each displacement detecting part 12 can detect the displacement of the vibration plate 6 in the Y direction by detecting the change in the capacitance corresponding to the change in the effective area between the electrode plates 8A and 10B.
As can be understood from equation 2, the detection sensitivity of the angular velocity sensor 1 having the above-described structure can be improved by increasing the vibration amplitude A of the vibration plate 6. The increase in the vibration amplitude can be achieved for example by increasing the voltage of the vibration driving signal applied to the vibration generating parts 11.
However, the maximum possible vibration amplitude A of the vibration plate 6 in the X direction is limited by the gap d0 between the electrode plates 8A and 10B of each displacement detecting part 12. Therefore, to increase the vibration amplitude A of the vibration plate 6, it is required to increase the gap d0. However, if the gap d0 between the electrode plates 8A and 10B is increased, the capacitance detected by each displacement detecting part 12 decreases in inverse proportion to the gap do.
As described above, if it is attempted to increase the detection sensitivity of the conventional angular velocity sensor having the above structure by increasing the gap d0, a reduction in the detection capacitance occurs. On the other hand, if the gap d0 is decreased, it becomes impossible to obtain a large vibration amplitude A. Thus, in the above-described structure employed in the conventional angular velocity sensor, it is difficult to improve the detection sensitivity by making a simple modification in the design parameters.